Aqueous solutions containing surfactants and so-called builder-substances, such as alkali hydroxides, alkali carbonates, borates, complexing agents, inorganic and organic corrosion inhibitors, and particularly silicates and/or phosphates are used in industrial degreasing and cleaning operations, particularly in the degreasing and cleaning of metal surfaces. During use they become contaminated with mineral oils and/or natural oils, which may contain emulsifiers, corrosion inhibitors, and other oil additives. The contaminants, which are emulsified by the emulsifier content of the oil and the surfactants in the cleaning solutions, diminish the effectiveness of the cleaning solutions and finally render them ineffective. The cleaning solutions must then be replaced. However, if the spent solutions are alkaline, they must be neutralized and oils must be removed in accordance with existing regulations before they can be discarded. Furthermore, neutralization alone will generally not result in adequate oil separation. Hence, means must be employed to break the emulsion which can be achieved by several methods, e.g. by lowering the pH of the solution to about 1; by treating the solution with salt; or by adding flocculating agents, such as aluminum or iron salts, in the acid range, followed again by neutralization. The components of the cleaning solution are of course lost when the solution, now free of oil, is discarded. Also, treatment of the waste water containing the discarded cleaning solution is generally required. Therefore, it is clearly advantageous to regenerate such spent cleaning solutions so that they can be re-used.
The regeneration of aqueous degreasing solutions can be achieved according to known methods either by means of ultrafiltration or by using centrifugal separators. However, when using centrifugal separators, only those degreasing solutions can be treated in which the oils and contaminants do not form stable emulsions. In addition, this process, like ultrafiltration, requires a considerable quantity of equipment, is time consuming, and is often energy intensive, making such regeneration processes relatively expensive. The reason why such processes have high equipment requirements is that in many instances the contaminants consist not only of organic substances but also of particles that are not soluble in water.